Defining River Corridors
FACT SHEET 
Vermont DEC River Management Program
Overview
A river corridor includes lands adjacent to and including
the course of a river. The width of the corridor is defined
by the lateral extent of the river meanders, called the
meander belt width (Figure 1), which is governed by
valley landforms, surficial geology, and the length and
slope requirements of the river channel. River corridors,
defined through ANR Geomorphic Assessments (2004),
are intended to provide landowners, land use planners,
and river managers with a meander belt width which
would accommodate the meanders and slope of a balanced or equilibrium channel, which when achieved,
would serve to maximize channel stability and minimize
fluvial erosion hazards.
Bw
Meander
Belt Width
Managing for Meanders
Building on the “fundamental principles of river systems”
and the diagrams of “floodplain access and channel evolution” laid out in River Corridor Protection and Management, Fact Sheet Q), this section will further explain
the components of channel geometry and why understanding their relationship with watershed function is essential to achieving the management objective of sustainable equilibrium river channels and avoidance of fluvial
erosion hazards.
Figure 1. Meander Belt Width (Bw) defined by the lateral
extent of meanders when the channel slope is in equilibrium
with the sediment transport requirements of the river.
Stable, equilibrium river channels erode and move in the landscape, but have the ability, over time and in an unchanging climate, to transport the flow, sediment, and debris of their watersheds in such a manner that they generally
maintain their dimension (width and depth), pattern (meander length), and profile (slope) without aggrading (building
up) or degrading (scouring down) (Rosgen, 1996; Leopold et. al, 1964). Stable, equilibrium rivers are considered a
reasonable and sustainable management objective in consideration of the repeated and catastrophic flood damages
experienced in Vermont. Many rivers are in major vertical adjustment due to human imposed changes in the condition of their bed and banks, slope and meander pattern, and/or watershed inputs (see Lane’s Balance in Figure 2).
Bed and bank resistance
Sediment input from
watershed
Establishing channel equilibrium as a river management objective, however, demands a recognition that
the geometry of certain river channels, due to their
location in the watershed, may be influenced by a net
storage or net export of sediment in the reach. In such
cases, the inherent vertical “instability” should be
assessed and potentially managed differently than the
river that is aggrading or degrading as a result of one
or more human imposed changes. For instance, it
may not be prudent to use the definition of stability
and manage against the aggradation which occurs on
an active alluvial fan, i.e. where streams transition
between steep mountain and gentle valley locations.
Also recognize that the potential level of achievement
of this objective may frequently be tempered by the
constraints of human investments on the landscape.
Slope and meander pattern
Water input from
watershed
Figure 2. Stable Channel Equilibrium (Lane, 1955)
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Protecting river corridors as defined by the meander belt width of the equilibrium channel avoids conflicts with
human land uses and minimizes investments and the need to conduct expensive channel management or stabilization
activities. Failure to recognize the physical imperatives of river systems and the land area that rivers will occupy
over time will demand large, on-going private and public expenditures to maintain an unsustainable condition of
dis-equilibrium which will ultimately fail.
Some Vermont rivers are presently in balance. The power produced
by flood flows and channel slope (a function of meander length) is
not so great as to cause significant scour (degradation) of the river
bed, or so diminished as to cause a loss of sediment transport capacity and a build up of sediment (aggradation) in the channel.
In these cases, it is cost effective to simply keep investments out of
the river corridor and avoid the eventual use of channel management practices, which become necessary to protect investments, but
ultimately change the river’s length and slope, lead to channel
adjustments, and increase erosion hazards.
For many Vermont rivers and streams, a combination of watershed,
floodplain, and channel modifications over the past 150 years, has
led to the major vertical channel adjustments that are ongoing today.
The initial stage of adjustment typically involved the bed scour and
head-cutting associated with channel straightening and degradation.
Steeper, straightened channels are now adjusting or “evolving” back
into more gentle gradient, more sinuous channels through an aggradation process (Figure 3). The narrower belt widths observed during Stages II and III of channel evolution, which held for decades
and encouraged human encroachment, have now begun to widen
during recent floods as new sediments deposit and longer meanders
develop putting human encroachments at risk.
The practice of dredging sediment to avoid flood hazards has typically worked until there is another flood. Berming and armoring
may hold longer, but tend to cause the unbalanced condition to extend upstream and downstream. Such practices are unsustainable
and will eventually unravel requiring extensive maintenance operations. A cost-effective, geomorphic approach would involve avoiding or minimizing encroachments and investments in river corridors. Corridors can be defined by applying fluvial geomorphic
principles to calculate and predict the belt widths which would
accommodate the meanders and slope of equilibrium river channels.
Defining the River Corridor
I
Belt
Width
I
II
III
Red = Narrower Belt Width
II
I
IV-V
III
Figure 3. A planform view of the Schumm (1984)
channel evolution model showing how adjustment
processes lead to a narrowing and then widening of
the meander belt width as the channel equilibrium
re-establishes at a more gentle slope.
When rivers are in dynamic equilibrium, a sustainable meander
geometry provides for the dissipation of the energy of moving water and the transportation of sediment. The fact that
unconfined, single thread streams tend to follow a sinuous or meandering course is related to the vertical (up and
down) oscillations of the stream bed. Flow characteristics (turbulence and secondary or lateral currents) cause the
selective entrainment, transport, and deposition of bed materials which produces systematic sorting of sediment sizes
between scour pools and riffle deposits. Riffles are the topographic high points in the undulating profile and pools are
the intervening low points. The combination and sequence of bed features results in converging and diverging flows
and leads to the development of a sinuous channel, with riffles becoming points of inflection (crossovers), where
the flow switches from one side of the channel to the other (Thorne, 1997).
2
Meander
Centerline
3x
Total
Belt
Width
6x
3x
Figure 4. Idealized representation of a river corridor
drawn to accommodate the meander belt width, measured
out as parallel lines “3 x channel width” either side a meander centerline drawn down valley through the crossover or inflection points of the river (dotted line).
Researchers have developed meander geometry formulas to
relate channel dimensions with planform measurements.
Williams (1986) using data collected from 153 alluvial
rivers around the world found that the relationship between
channel width and the meander belt width is expressed by
the formula B=3.7W1.12 (where B is the belt width and
W is the channel width in feet for channels ranging from
5 to 13,000 ft wide). This formula results in a meander
width ratio approximately equal to six (i.e., the belt width
is equal to about 6 bankfull channel widths). Corridors for
gentle gradient rivers and streams (slope < 2%) in narrow
to broad alluvial valleys are calculated and drawn to
accommodate a meander belt width that is equal to 6 times
the width of the river channel.
Where rivers are assessed as being in equilibrium and the lateral extent of their meanders create a belt width that is at
or near the “6 times channel width” relationship, then corridors are drawn as two roughly parallel lines, following
down the valley and capturing the extent of existing meanders (Figure 4). If the river slope and sinuosity have been
modified, the corridor is drawn using 3 channel widths either side of a meander centerline or 6 channel widths out
from the toe of the valley if the river is presently flowing less than 3 channel widths from the toe (Figure 5)
Rarely does one find the idealized sinuosity shown in Figure 4.
Rivers and streams in Vermont are usually less sinuous, many having
been straightened against a valley side slope. In these cases, the
river corridor (still “6 times channel width”) is drawn so that the belt
width extends laterally out from the valley toe (see Figure 5). These
corridors are not established with the expectation that river adjustments will occur and result in a perfect sine wave pattern which conforms to the calculated belt width. Rather, they provide an area
within which channel adjustments may occur, in order to re-establish
an equilibrium condition, and there can be a reasonable expectation
that fluvial erosion hazards will be minimized.
Figure 5 illustrates a river corridor, in a broad gentle gradient valley,
which was drawn using a combination of river and valley features.
The river starts out against the left valley wall (Segment A), flows
across the valley (B), returns to the right valley wall (C), flows
through a set of meanders (D), and then again along the left valley
wall (E). All but Segment D represent the planform of a river reach
which has been historically straightened. The meander centerline
(red dashed line) travels between meander crossovers where they
exist but otherwise, follows the path of the river. The river corridor
is a belt width (solid black lines) equal to 6 times the channel width;
3 widths either side of the centerline in Segments B and D, and 6
widths out from the toe of the valley in Segments A, C, and E.
The River Management Program has developed GIS extension software, called the Stream Geomorphic Assessment Tool (SGAT), to
automate the process of creating river corridors, once the geographical features: streams, valley walls, and meander centerlines are
defined.
3
E
D
Toe of left
valley wall
Toe of right
valley wall
C
B
A
Flow
Figure 5. River corridor drawn for a reach of river
straightened against the toe of the valley.
Adjusting Corridor Widths
Belt widths “6 times the channel width” develop on rivers which are gentle-sloped, unconstrained, and have erodible
boundaries. Obviously, these conditions do not prevail in all Vermont valleys and there are both geographical and
human constraints that may justify changing river corridor widths and locations, including:
Y Existing private investments and public infrastructure for which there is a longer-term public commitment to protect (armor) against fluvial erosion hazards (e.g., town and state roadways);
Y Steeper, confined to narrow valleys with less erodible boundaries, where corridors of “1 to 4 times
channel width” are recommended based on stream type and specific valley characteristics; and
Y Extremely sensitive stream types or landslide areas that may require corridors > 6 channel widths.
Refer to the “Technical Guidance for Determining Floodway Limits” (ANR, 2003) for more information on adjusting
river corridors by stream and valley type and accommodating human developments and infrastructure.
Practical Planning and Management Tool
Defining river corridors is essential to the development and implementation of river corridor plans. Such plans should
include a process for selecting and implementing river corridor management alternatives and providing a basis for
corridor protection through various land use planning and incentives programs. River corridors can define flood
hazard zones or overlay districts thereby supporting implementation of town pre-disaster mitigation plans, or be incorporated into the watershed (basin) plans developed by regional, state, and federal agencies. River corridors defined
and “adopted” as part of a public process become a practical, science-based planning tool for directing the use
of public funds to reduce fluvial erosion hazards.
River corridor plans, while setting objectives for managing toward a geomorphically-stable river and reducing fluvial
erosion hazards, should also recognize that nearly all landowners have made some investment in their lands along a
river. Adopting a river corridor plan would not necessarily require the removal of existing investments, but rather
would work to avoid future encroachments within the meander belt width which eventually require long-term commitments to bank armoring and other channelization practices for their protection. To deal with conflict areas, for
instance when the channel lengthening process threatens an existing investment either within or at the bounds of the
corridor, the plan would spell out a range of alternatives and a process for resolving conflicts. At one end of the
range, the plan would create the opportunity for willing landowners to be appropriately compensated for removing
investments and changing land uses within the corridor. On the other end, the plan may recognize certain reaches
where, for example, transportation infrastructure is located and keeping the river channelized is in the public interest.
Implementing river corridor plans will require a long-term commitment to reducing fluvial erosion hazards and
restoring the natural and recreational values of rivers, while respecting traditional settlement patterns and the importance of a prosperous agriculture in Vermont. From one decade to the next, opportunities arise to work with landowners in a cooperative fashion, increasingly if not gradually giving the river more space to achieve equilibrium.
Without a corridor plan, encroachments will continue, compounding the cost of flood recovery, and necessitating
river management that is both economically and ecologically unsustainable.
References
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8.
VT ANR. 2004. Stream Geomorphic Assessment Protocol Handbook.
www.watershedmanagement.vt.gov/rivers.htm
VT ANR. 2003. Technical Guidance for Determining Floodway Limits. www.watershedmanagement.vt.gov/rivers.htm
Lane, E.W. 1955. The Importance of Fluvial Morphology in Hydraulic Engineering. Proceedings of the American Society of Civil Engineers, Journal of the Hydraulics Division, vol. 81, paper no. 745.
Leopold, L.B., M.G. Wolman, and J.P. Miller. 1964. Fluvial Processes in Geomorphology. Freeman, San
Francisco, 522pp.
Rosgen, D.L. 1996. Applied River Morphology. Wildland Hydrology. Pagosa Springs, CO.
Schumm, S.A. 1984. The Fluvial System. John Wiley and Sons, New York.
Thorne, C.R., R.D. Hey, and M.D. Newson. 1997. Applied Fluvial Geomorpholgy for River Engineering and
Management. John Wiley and Sons, Chichester, UK.
Williams, G.P. 1986. River Meanders and Channel Size. Journal of Hydrology 88:147-164.
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